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Old 11-03-2007, 05:51 PM   #1
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looking for aero suggestion

Hey there, what we have is a saab sonett III. that I am slowly rebuilding (opposite college). Well when I bought a part car I was lucky enough to get a spare fiberglass body as well . Hmm one stock body AND one low CD body. Does anyone have any suggestions? I plan on doing some scale model and wind tunnel tests.

the top pic should be stock and lowered

the bottom pic should be my first idea
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Old 11-03-2007, 07:45 PM   #2
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Nice drawings!

Be sure to read the sticky at the top of the aero mods section. Lots of good information there.

I think you're talking about adding:
front air dam (usually good, especially if you add side skirts)
hood scoop (increases drag)
side skirts (works together with air dam)
ultra low profile wheels (not an aero thing really)
wheel skirts (good idea)
boat tail (very good idea if you have the skills etc.)

Re. boat tail, search for posts here by basjoos and then look up his garage entry. He's the boat tail king so far as I know.

Ultra wide, ultra low profile tires probably don't do much for fuel economy. Most of the cars with really good fuel economy (stock) have skinny tires stock. I don't have any other reference on that but I haven't seen much talk here about ultra wide/low tires.
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Currently getting +/- 50 mpg in fall weather. EPA is 31/39 so not too shabby. WAI, fuel cutoff switch, full belly pan, smooth wheel covers.

Now driving '97 Civic HX; tires ~ 50 psi. '89 Volvo 240 = semi-retired.
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Old 11-03-2007, 08:22 PM   #3
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Thanks I am not into those low pro's either, my bud drafted up the origional, he is deep into the ricer style. Then I added the body mods, Yea I looked and found Basjoos's -> really the inspiration.

The hood scoop is required because the ford v-4 engine actually pops up above the panels. I was thinking however that a little foamboard and some work. To turn it a hood bump shaped like a cowl induction would clear that up.

But other than that, do the lines I drew up look right? I'll probably end up making several variants and analyze for the best. Thanks again
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Old 11-03-2007, 09:04 PM   #4
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Hopefully basjoos will write in sooner or later.

I remember reading this - maybe someone can confirm or quote - a long sloped nose is better than a front air dam. I have an air dam because I was able to build it. I believe a long nose with gentle slope would be better but it's not a project I can do.

Air dam's benefit is that it prevents the air from making trouble underneath the car. Air dam creates a relative vacuum down there, especially if side skirts are used. HOWEVER you are doing all the work of pushing the air out of the way and maintaining the vacuum.

Much better to have a long gently sloped nose reaching down pretty far. That way the air is gently directed upwards and flows on a smooth path to the windshield and then over the top. If you'll see basjoos' pics that's what he's had for a while now.

Most cars have a nose designed so that if you want to make a long sloped nose, you need to extend the existing nose forward quite a distance. You will have to see what would be needed on your Sonett.
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Currently getting +/- 50 mpg in fall weather. EPA is 31/39 so not too shabby. WAI, fuel cutoff switch, full belly pan, smooth wheel covers.

Now driving '97 Civic HX; tires ~ 50 psi. '89 Volvo 240 = semi-retired.
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Old 11-04-2007, 03:50 AM   #5
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I always like those Sonnet III's and the Opel GT's (Mini Vette)
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Old 11-04-2007, 08:34 AM   #6
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The cowl idea would be better than the scoop for sure. Can you get some pictures of the actual cars?
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Old 11-04-2007, 09:23 AM   #7
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Thanks for the suggestions. As far as the front one piece goes anything forward of the windshield is sacrificial. There is an incredible amount of room. As long as a hood popup is there for the V-4's height. Until I swap in a shorter motor maybe a diesel.

Oh, and I have though about roof chopping. But given I am 6'3" and the car is only 3'10" tall. Not the mention that shortened glass would be hard to locate. I don't think it is legal around here to use polycarbonate sheets.
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Old 11-04-2007, 11:22 AM   #8
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Quote:
Originally Posted by sonettIII View Post

Oh, and I have though about roof chopping. But given I am 6'3" and the car is only 3'10" tall. Not the mention that shortened glass would be hard to locate. I don't think it is legal around here to use polycarbonate sheets.
I'm not sure how common they are, but you can have a glass maker that can handle windshield laminates "cut" the windshield We used a company called Glass Benders - which may be a chain (not sure). But their specialty was marine applications - mostly because they were very near to a large marina
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Old 11-05-2007, 01:03 PM   #9
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Well today I got around to building my model wind tunnel. Pieced together out of a HUGE surplus furnace blower, 1/2 hp electric motor and some plywood.But it is not quite perfected yet. I still need to figure out some smoke generator. Right now I am trying a oil lamp with mineral spirits. I'll see how that works. And I also made up some car profiles with some blueboard.
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Old 11-05-2007, 06:01 PM   #10
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If you have smooth underbody paneling under your car, then you don't need an air dam, since the main purposes of an air dam is to divert air to keep it from coming into contact with the rough underside typical of most cars and to produce a pocket of low pressure under the car for high speed driving. But the air dam adds frontal area, which hurts FE, and for FE driving you aren't all that concerned about the extreme high speed cornering that the low pressure under the car would help with at the expense of FE. For ultimate FE, I would go with a smooth underpaneling rather than the air dam.

The whole idea of the long sloped nose is that it puts the air-splitting leading edge of the car as low as possible so that most of the air hitting the nose goes smoothly over the car, rather than under the car where it adds to the turbulent drag under the car. Adding the nosepiece to my car noticably improved it's coasting ability and the change in the air flow greatly reduced the number of insects hitting my windshield (and has saved the lives of several birds that are picked up and carried over the car by the airflow rather than hitting the grill like they did before I modified the nose.

Looking at the photos of the Sonett, the main aero defect I notice is the leading edge of the nose is way too high (and flat), so that much of the oncoming airflow is diverted under the car. If you reshape the nose, you can integrate the reshaped nose with a grill block or adjustible radiator inlet door. Also there are some steps at the top of the hatch and behind the door that could be smoothed out. It looks like it would be relatively easy to put a boattail on it. Also it would be worthwhile to reposition the windshield wipers to the bottom of the windshield, and place them behind a windscreen so that they don't add to the drag.
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